Who is the will of the abyss?

The Will of the Abyss is Alice's twin. In the manga, both Alice and The Will of the Abyss are born of Lacie(Oswald's younger sister) and Levi. In the anime, the birth of the two girls is not explained, nor is it really clarified whether or not they are two separate people or if they're two souls joined into one body. The Will of the Abyss embodies The Core of the Abyss(the deepest level). The Will of the Abyss looks exceptionally like Alice. The main distinction between the two is that The Will of the Abyss has white hair.